Mortise and tenon joinery can be used for many applications in the log home industry. These applications range from exterior deck railing to interior staircase and loft railing, to dining room and bedroom furniture.
The steadily growing log home industry, in 2003, had over $1.5 billion in sales and has more than 550 manufacturers in North America alone. An estimated $70-$100 million of these sales are composed of small diameter log joinery. The process of creating this joinery however is very labor and layout intensive and requires skilled craftsmen to perform the work. The current method of constructing custom log joinery involves making each component individually.
In the interest of continuity the following discussion is limited to the production of log railing. The concepts discussed can however be extended to the production of furniture as well.
The components of a log railing consist mainly of the following: 1) vertical posts providing the interface between the railing and the deck; 2) horizontal rails which connect the posts; and 3) spindles.
A key aspect in providing a professional looking product is ensuring a tight fit at the interface between any two components. This tight fit involves mating a conical surface and a cylindrical surface and therefore requires the axial dimension of each component to be based on the surface separation of the two elements between which it is to be joined. This surface dimensioning system is fundamentally variable due to the taper and curvature of a natural log.
The existing method of building log railings or furniture uses a surface dimensioning system. This requires a process as follows: 1) Layout and install the posts complete with pre-drilled mortises; 2) Measure the surface-to-surface length of the top and bottom rails—these dimensions will differ due to the taper and other irregularities of the posts; 3) Allow for the tenon length, which extends into the mortise beyond the surface interface, cut the rail material to length and drill the rail tenons; 4) Layout and drill the rail mortises; 5) Install the two rails in the posts, or some other jig which can spread the rails to some predetermined parallel; 6) Measure the surface-to-surface length of each spindle, which will all differ due to the irregularity in the material; 7) Add the tenon length to each of these individual spindle dimensions and cut each spindle to length; 8) Cut the tenons on each spindle; 9) Disassemble rails from posts (or jig) and install the spindles into the rails; 10) Install the whole rail-spindle assembly into the posts; and 11) Re-secure the posts to the sub-framework.
This surface dimensioning system implies that each post, rail and spindle produced, fit in only one individual position in the railing. This custom process proves to be far too inefficient for industrial production. Consider for example a 150 ft railing, around an exterior deck, which could consist of up to 20 Posts, 40 Rails and 400 Spindles. Unless the job is done on site, the accounting nightmare is considerable.
In order to industrialize the process, some companies have turned the logs into perfect cylinders ensuring constant predicable surface dimensions. This has however increased the production costs and eliminated the custom and natural look of the product, thereby decreasing the retail value and subsequently the overall return.
There is therefore a need for a machine which, using naturally occurring irregular material, can produce custom looking railing or furniture in a modular industrial fashion.